An optical disc apparatus capable of recording and/or reproducing information on/from an optical disc such as a BD (Blu-ray Disc), a DVD (Digital Versatile Disc) or a CD (Compact Disc) has been widely spread. In order to read information from the optical disc, focusing control and tracking control for causing an objective lens supported in an optical pickup to follow a face wobbling (wobbling of the optical disc in a focusing direction) and an eccentricity (wobbling of the optical disc in a tracking direction) generated during rotation of the optical disc are required. The face wobbling component and eccentricity component can be expressed as sine waves whose one period is the time required for one rotation of the optical disc and whose amplitudes are the face wobbling amount and the eccentricity amount respectively. In the following description, control of causing the objective lens to follow the eccentricity of the optical disc will be mainly explained.
Though the objective lens vibrates at its own natural vibration frequency in a tracking-off state in which no tracking control is performed, it is assumed here that the objective lens is at rest in a center position of an optical pickup supporting the objective lens. Under this assumption, the optical disc alone becomes eccentric during rotation of the optical disc and a track of the optical disc crosses the objective lens. However, it can be seemingly regarded that the objective lens moves in a tracking direction (that is, in a radial direction of the optical disc) to cross the track of the optical disc.
Usually, in an optical disc apparatus, after a period (track crossing period) TCR (sec), in which the objective lens crosses the track of the optical disc during rotation of the optical disc, has become larger than a predetermined period threshold value TTH (sec), tracking pull-in is executed. During rotation of the optical disk, a velocity VCR (m/sec) (track crossing velocity), at which the objective lens crosses the track of the optical disc, is proportional to the reciprocal of the track crossing period TCR (sec). Thus, the aforementioned control content is equivalent to that the tracking pull-in is executed after the track crossing velocity VCR (m/sec) has become smaller than a predetermined velocity threshold VTH (m/sec).
Letting a maximum value of an optical-disc eccentricity velocity VE(t) during rotation of the optical disc be VEMAX (m/sec), the maximum value VEMAX (m/sec) of the eccentricity velocity VE(t) is extremely larger than the velocity threshold VTH (m/sec) (VTH<<VEMAX) in general. Here, the eccentricity velocity VE(t) is a relative movement velocity in the radial direction between the optical disc and the objective lens during rotation of the optical disc. Hence, a timing of execution of the tracking pull-in is a point of time at which the optical-disc eccentricity velocity VE(t) becomes substantially zero. This point of time is a time when an optical-disc eccentricity component Ecc(t) becomes a maximum or a minimum (that is, a time when the eccentricity amount which is the absolute value of the eccentricity component becomes a maximum). For example, if the tracking pull-in is executed at the time when the optical-disc eccentricity component Ecc(t) becomes a minimum, since the objective lens needs moving, with respect to the center position of the optical pickup, by an eccentricity amount from the minimum to the maximum of the optical-disc eccentricity component Ecc (i.e., Peak-to-Peak amount of the eccentricity amount), a lens offset amount of the objective lens after the tracking pull-in is large. If the eccentricity amount of the optical disc is large, the lens offset amount further increases. If the lens offset amount is too large, there is a risk that the tracking control causes the objective lens to move beyond an allowable movable range in the tracking direction, and there is another risk that not only it makes tracking control unstable but also causes damage to the objective lens.
Proposals for tracking control are described in Patent References 1 and 2, for example. Patent Reference 1 proposes a tracking control device which measures an eccentricity amount of the optical disc (a wobble width in the tracking direction of the optical disc) before tracking pull-in and outputs a drive signal for correcting the eccentricity amount on the basis of the measurement result. Patent Reference 2 proposes an optical disc apparatus which acquires a plurality of lens offset amounts from a tracking drive signal corresponding to one rotation of an optical disc and adjusts by using their representative value a sled position which is a position of an optical pickup in the optical-disc radial direction.